1. Field of the Invention
The invention relates to a process for the joint removal of ammonia and lower alkanes and/or hydrogen from offgas streams in an industrial plant by combined oxidation and reduction. Here, the content of the lower alkanes and/or hydrogen in the same or a further offgas stream of the industrial plant should be partly utilized as reducing agent for the nitrogen oxides liberated in the oxidation of ammonia, with the concentration of both the ammonia and the lower alkanes being decreased. In this novel process, the pollutants which are in any case present in the process offgases thus serve as reaction partners for efficient removal of one another. This reduces the outlay compared to separate purification of the respective offgas streams.
2. Description of the Related Art
In some industries, e.g. in the production of fertilizers, offgas streams contaminated with ammonia or with lower alkanes, for example methane, are obtained in various sections of the process. Owing to the environmental and odor pollution brought about by such compounds, these offgas streams have to be purified before release into the ambient air. These offgas streams are, for example, the offgas streams from the low-pressure absorber or the atmospheric absorber in a urea synthesis plant. Such offgas streams are problematic since their emission values of the pollutants present therein are far above the recommended concentrations proposed in the technical guidelines for clean air. See First general administrative regulations relating to the German federal environmental protection law, 2002. Here, for example, a value of 50 mg/standard m3 is required for ammonia emissions.
Many processes for the separate purification of ammonia- or alkane-containing streams are now prior art and are presented below. The term nitrogen oxide used here is a conventional collective term for the gaseous oxides of nitrogen, e.g. nitrogen monoxide NO, nitrogen dioxide NO2 (together referred to as NOx) or dinitrogen monoxide N2O (nitrous oxide).
The removal of ammonia from exhaust air is achieved by addition of sulfuric acid or nitric acid in the processes known in the patent literature. This process is also known under the name “acid scrub”, as described, for example, in U.S. Pat. No. 3,607,022. The ammonia is in this case removed from the exhaust air stream by chemical absorption, with ammonia being converted into the corresponding ammonium salt in the acidic solvent. Thus, the use of nitric acid leads to formation of ammonium nitrate, while ammonium sulfate is formed when sulfuric acid is used. These solutions containing ammonium salt then form wastewater streams which cannot simply be released into the wastewater system. A solution to the disposal of the salt solution produced thus has to be found. In addition, the acid scrub requires a large outlay in terms of apparatus and logistics since, apart from the complex scrubber, appropriate amounts of acid also have to be purchased and stored.
Furthermore, the prior art for the disposal of ammonia-containing offgas streams includes combustion in flares. However, significant amounts of auxiliary gas are required to ensure a minimum calorific value, depending on the concentration of ammonia and other combustible impurities. The combustion of ammonia taking place in an uncontrolled manner in the flare also leads to formation of significant amounts of nitrogen oxides which likewise represent environmental pollutants and whose formation should thus be avoided. In addition, open flares are considered to be problematic for continuous operation by the authorities in Europe because of the lack of possible emission measurements. On the other hand, the operation of an alternative, closed flare which allows controlled combustion with subsequent emission measurement is associated with a higher outlay in terms of apparatus.
A further process mentioned in the patent literature for purifying ammonia-containing offgas streams is selective oxidation of ammonia to form nitrogen, which is carried out with the aid of specific catalysts and is described, for example, in DE 695 30 024 T2 and EP 0 514 729 B1. This oxidation leads to evolution of heat which in the case of uncooled reactors leads to an increase in the reactor temperature. However, the selectivity of this process decreases at increased temperatures, i.e. the proportion of nitrogen oxides formed increases. Thus, this process is usually used for streams which are contaminated with small amounts of ammonia and lower alkanes and/or hydrogen.
EP 1 350 552 B1 discloses a process for the treatment of ammonia-containing streams with the aid of the SCR (selective catalytic reduction) technology. In a first step, the combustion of the ammonia takes place in a thermal after-incineration plant, with nitrogen oxides inevitably being formed. The invention then makes use of the fact that the combustion of ammonia does not proceed completely and provides for the unburnt ammonia to be utilized in a further step in order to destroy the nitrogen oxides by means of selective catalytic reduction. Complicated regulation has to be expected in this process since the degree of combustion has to be matched to the amount of nitrogen oxides formed.
A further alternative for the removal of ammonia from offgases is adsorption, e.g. on activated carbon or zeolites, as disclosed in U.S. Pat. No. 6,261,345 B1. A disadvantage of this process is its non-steady-state operation consisting of adsorption and desorption phases, which increases the outlay both in terms of apparatus and for operation.
The physical absorption of ammonia using solvents such as water or glycols is likewise prior art, as evidenced by U.S. Pat. No. 5,230,877. However, the purities of the offgas which can be achieved here are greatly dependent on the operating conditions of the scrub, e.g. temperature and pressure. High pressure and low temperature are advantageous, but these result in additional capital and operating costs. Absorption in water is used, for example, for the gas mixture consisting of ammonia, methane and nitrogen obtained as purge or flash gas in the synthesis of ammonia. Here, in particular, the high admission pressure of the gases (about 200 bar) is advantageous for physical absorption.
The lower alkanes which likewise occur in offgases belong to the group of volatile hydrocarbons. Particular mention may be made of methane which is a strong greenhouse gas which is up to 20 to 30 times more damaging than carbon dioxide.
A simple method of purifying the alkane- and/or hydrogen-containing offgas streams is thermal utilization. However, when proportions of ammonia are present, the problems of nitrogen oxide emission due to incomplete or unselective combustion have to be taken into account again. In addition, if calorific values of the offgases are too low, additional fuel such as natural gas has to be consumed in order to ensure stable combustion.
A reduction in the fuel required for oxidation of organic constituents can be achieved by the use of regenerative catalytic after-combustion. Here, a reduction in the combustion temperature required is achieved by the presence of a suitable noble metal catalyst. Furthermore, the heat evolved as a result of combustion is utilized to preheat the untreated crude gas via a plurality of reactor beds which can be connected differently. Owing to the non-steady-state operation of the individual reactors with heating and cooling phases, complex connection and regulation of a plurality of reactors is necessary for industrial use, which makes the use of this process costly.
The purification of an offgas stream which is contaminated both with ammonia and with lower alkanes is possible only with difficulty according to the available prior art. This is because, according to the available prior art, various purification steps have to be combined in a sequence in order to meet the required limit values, especially for the nitrogen oxides, in the offgas.